1. Field of the Invention
This invention relates to a fuel cell, and more particularly, mainly concerns the construction of a bipolar plate, a manufacturing method of the bipolar plate and the like.
2. Description of the Prior Art
FIG. 1 is a perspective view showing a conventional fuel cell bipolar plate, which is illustrated in the Japanese Unexamined Patent Application Published under No. 208559/87 (Sho. 62) and referred to as a separator plate in that publication, in the state of being separated by each plate member on its upper side. In that figure, reference numeral 1 designates a bipolar plate substrate which separates fuel gas and oxidant gas in the laminated direction. Reference numeral 2a designates a first hard frame in a sealing part, which is referred to as a distance piece in the above publication, for forming a gas seal between layers and fuel supplying or exhausting manifolds; numeral 2b designates a second hard frame, which is referred to as a mask in the above publication, for forming a gas seal between layers similarly; numeral 3 designates a corrugated plate for maintaining fuel channels to be formed in the hollowed part 8 at the inside part of the hard frame 2a; numerals 4 designate oxidant gas supplying manifolds; numerals 5 designate fuel gas supplying manifolds; numerals 6 designate oxidant gas exhausting manifolds; numerals 7 designate fuel gas exhausting manifolds; and numeral 9 designates a hollowed part at the inside part of the hard frame 2b, in which hollowed part 9 an electrode (not shown) is to be inserted. On the hard frame 2b an electrolyte matrix, which has the same shape as that of the bipolar plate substrate 1, is furthermore laminated, and then a hard frame 2b, a corrugated plate 3, a hard frame 2a and a bipolar plate substrate 1 are respectively laminated on the electrolyte matrix in this order so as to be symmetric to the lower part with respect to the electrolyte matrix. In the upper hard frame 2a corresponding to the lower hard frame 2a, the oxidant gas supplying manifolds 4 and the oxidant gas exhausting manifolds 6 are respectively opened to the hollowed part 8, and conversely the fuel gas supplying manifolds 5 and the fuel gas exhausting manifolds 7 are isolated. The required number of cells are laminated by repeating the above described lamination, and appropriate surface pressure is impressed on the top and bottom surfaces of each layer having been laminated.
FIG. 2 shows an enlarged view of a cross section A--A, illustrated in FIG. 1, of the laminated fuel cell, which place is a gas sealing around electrode planes. In the figure, reference numeral 20 designates an oxidant gas side electrode; numeral 22 designates a corrugated plate for oxidant gas; numeral 24 designates an oxidant gas side current collector; numeral 25 designates an anode electrode; numeral 26 designates a fuel gas side current collector; and numeral 30 designates an electrolyte matrix.
Next, its operation will be described. Fuel gas is supplied into the hollowed part 8 through the fuel gas supplying manifolds 5 which penetrate through the bipolar plate substrate 1, the hard frame 2a and the hard frame 2b vertically, and the fuel gas passes through the corrugated plate 3, performing cell reactions, to be lead to the fuel gas exhausting manifolds 7. On the other hand, the oxidant gas passes through the channels on the opposite side of the cell from the oxidant gas supplying manifolds 4, which similarly penetrate the substrate 1 and the hard frames 2a, 2b vertically, through the electrolyte matrix 30 to be lead to the oxidant gas exhausting manifolds 6, contributing to cell reactions.
In the construction of the conventional fuel cell bipolar plate, since the gas sealing parts around the electrode planes and the gas sealing parts of the manifolds adjoin on the same plane, electrolyte matrices which are filled up with electrolyte are used as the gaskets of the gas sealing parts of the manifolds similarly to the cell reacting area, too. That is to say, in the above conventional construction, an electrolyte matrix which has the same size as the external shape of the hard frame 2b is used as the electrolyte matrix 30, and the electrolyte matrix 30 has a performance of the gasket materials in the sealing parts around the manifolds, also. The electrolyte matrix 30 holds electrolyte in the vacant spaces of a porous electrolyte-holding material, and has the gas sealing function. The electrolyte matrix 30 is directly held on both of its surfaces between the hard frames 2b made of metal material at the gas sealing parts around the manifolds, and it has no electrolyte supplying source in the close vicinity. Accordingly, electrolyte permeates through the electrolyte matrix 30 from its cell reacting area to be supplied to its manifold parts.
In the prior art bipolar plate shown in FIG. 2, the hard frames 2a and 2b form gas seals around electrode planes together with the electrolyte matrix 30, and the height of its gas sealing part and the height of its cell reacting area, both of which are expressed by the next formulae (1) and (2) respectively, are designed so as to be substantially the same, and thereby the occurrences of cracks on the boundary line between gas sealing area and reacting area of the electrolyte matrix 30 are prevented. ##EQU1##
Since the conventional fuel cell bipolar plate has the above construction and operates as mentioned above, there is the possibility of occurrence of the leakage of gas caused by the appearance of vacant spaces between layers in some combination of the thickness accuracy, the surface roughness, the distortions and the like of the bipolar plate substrate 1, the hard frame 2a and the hard frame 2b. Furthermore, since the sealing parts are constructed of the hard frames, the surface pressure which is impressed on the top and the bottom surfaces of each layer of the bipolar plate may produce the difference in magnitude between the sealing part and the reacting area of the electrode. Furthermore, since the fuel gas supplying manifolds 5, the fuel gas exhausting manifolds 7, the oxidant gas supplying manifolds 4 and the oxidant gas exhausting manifolds 6 are contacted with each other through the bipolar plate substrate 1, the hard frame 2a and the hard frame 2b, all of which are made of metal, and thereby the above construction makes it easy to wet the surfaces of each layer at the above gas sealing part around the manifolds due to the electrolyte oozing out of the electrolyte matrix 30; local cells are easy to be occurred on the metal surfaces around the manifolds, and consequently the conventional fuel cell bipolar plate has a problem that it can not operate for a long term stably in high output power.
Also, since the conventional fuel cell bipolar plate uses the hard frames as the gas seals, it is difficult to hold the sufficient amount of electrolyte for the gas seals around the manifolds in the close vicinity of the electrolyte matrix 30 at the gas sealing part, and then it is required to supply the electrolyte from the cell reacting area of the electrolyte matrix 30. Then, the electrolyte held at the cell reacting area of the electrolyte matrix 30 and held at the electrode part is shared by the electrolyte matrix 30 at the cell reacting area and the gas sealing part.
Consequently, the conventional fuel cell bipolar plate has problems that initially overplus electrolyte must be held in the cell reacting area, which damages the initial performances of the cell; that the utilizable amount of electrolyte in the cell reacting area is limited to shorten the life of the fuel cell bipolar plate; and that electrolyte is needed to move from the cell reacting area to the gas sealing part through the electrolyte matrix 30, which makes it difficult to fill up electrolyte quickly and attain the gas sealing performance, although they are preferable at the initial time after heating up of the stack.
Besides, in the conventional construction of the bipolar plate, since the gas sealing parts around the manifolds and the gas sealing parts around the electrode planes are in the close vicinity, it is difficult to use the widely used gasket materials whose principal ingredients are silica, alumina, talc and the like as the gaskets at the gas sealing parts around the manifolds for preventing the occurrence of the corrosion caused by the electrolyte having oozed out of the gas sealing parts around the electrode. Accordingly, the electrolyte matrix 30 which contains electrolyte having high reactivity in the aspects of the electrochemistry and the corrosion of materials must be used as the gasket, then the conventional construction of the bipolar plate has such problems in the aspect of the stability of gas sealing for a long term as the decrease of the wet sealing ability and the corrosion of the materials around the gasket, both of which are caused by the migration of the electrolyte to other places of the laminated cell as a result of the electrochemical reaction.
Also, the conventional construction has a tendency to produce the difference in height between the gas sealing part and the cell reacting area, and it is substantially extremely difficult to level the height. One of its reasons is that tolerances for manufacturing, for example about .+-.0.02.about.0.06 mm, are required for each member when the members of the bipolar plate such as the bipolar plate substrate 1, the hard frame 2a, the hard frame 2b, the oxidant gas side electrode 20, the fuel gas side electrode 25, the current collectors 24 and 26, and the corrugated plates 3 and 22 are manufactured, then it is very difficult to level the height at the gas sealing part and the cell reacting area in the aspects of manufacturing cost and technology owing to the accumulation of the tolerances. Another reason is that it is difficult practically and technically to level the height of both the parts always even when changes with the passage of time occur, because the electrodes 20 and 25 have the tendency of easily changing in thickness with the passage of time, for example the fuel gas side electrode 25 reduces in thickness from 10 to 20 .mu.m for 10,000 hours in a typical condition.
Since such differences in height to occur inevitably exist on the boundary lines between the gas sealing part and the cell reacting area on both sides of the fuel gas side and the oxidant gas side through the electrolyte matrix 30, the conventional construction of the bipolar plate has a problem that cracks will easily occur in the electrolyte matrix 30 along the boundary line and thereby the fuel gas and the oxidant gas are mixed.
Next, another prior art for resolving the unevenness of the surface pressures between the gas sealing part and the cell reacting area will be described. FIG. 3 is a partly broken perspective view showing the conventional fuel cell bipolar plate which is illustrated in the Japanese Unexamined Patent Application Published under No. 75162 / 90 (Hei. 2), and in which the soft frame around the electrode plane and the soft frame around manifolds are constructed with the bipolar plate substrate in a body to form a sealing plane. In the figure, reference numeral 1 designates a bipolar plate substrate; numeral 4 designates an oxidant gas supplying manifold (or a supplying aperture); numeral 5 designates a fuel gas supplying manifold; numeral 6 designates an oxidant gas exhausting manifold; and numeral 7 designates a fuel gas exhausting manifold. On the fuel side where the fuel gas is supplied to a cell reacting area 12, only an outside soft frame 10, which intercepts gas, is attached at the outside of the fuel gas supplying manifold 5 and the fuel gas exhausting manifold 7; and on the opposite side of the cell reacting area 12 of the bipolar plate substrate 1, an inside soft frame 11 is attached together with the outside soft frame 10 for not supplying fuel gas to the cell reacting plane.
In the present bipolar plate constructed as described above, since the same member as the cell reacting plane is inserted into the soft frame, there happens no unevenness between the surface pressures of the sealing part of the soft frame surfaces and the cell reacting plane. But, since the fuel gas and the oxidant gas are partitioned by the inside soft frame 11, the electrolyte having oozed out of the sealing parts reacts to produce local cells, and thereby the corrosion of the soft frame is accelerated. Besides, from the view point of the construction, since it is required to weld the three members of the bipolar plate substrate 1, the inside soft frame 11 and the outside soft frame 10 respectively, the construction has a problem that processing is difficult and manufacturing cost is high.